1. Field of the Invention
This invention relates generally to a modulated clock minimum shift keying (MSK) modulation technique for transmitting digital data and, more particularly, to a modulated clock MSK modulation technique for transmitting digital data in which the digital data modulates or selectively switches a clock signal prior to the data being modulated onto a carrier wave signal for transmission.
2. Discussion of the Related Art
Transmission of digital data at a high data transmission rate is necessary for many types of digital communications systems. Systems requiring high digital data transmission rates include video transmission systems, satellite communications systems, etc. In these systems, digital data is generally modulated for transmission by an applicable modulation technique, such as phase modulation. Phase modulation involves modulating a carrier wave signal where the phase of the modulation wave distinguishes the "0" and "1" bits.
Minimum shift keying (MSK) modulation is one type of phase modulation applicable for high digital data transmission rates. MSK modulation involves separating the digital data into an in-phase (I) data stream and a quadrature (Q) phase data stream. The I and Q digital data streams are passed through a sinusoidal pulse shaper and then separately mixed with a carrier wave signal at the particular phase of the I and Q data streams before being recombined to be transmitted. Separating the digital data into separate modulated data streams with carriers 90.degree. out of phase with each other allows more digital data to be transmitted within a given time interval. A background discussion of different types of MSK modulators can be found in U.S. Pat. Nos. 5,216,391 issued to Shiraishi et al.; 5,020,079 issued to Vancraeynest; and 4,500,856 issued to Childs.
One type of modulation related to MSK modulation is referred to as staggered quadrature phase shift keying (SQPSK) modulation. For SQPSK modulation, the I and Q modulated data streams are separated in phase by 90.degree., and one of the data streams is staggered by a one-half bit time. SQPSK modulation is distinct from MSK modulation mainly because square wave I and Q digital data signals are applied to the carrier wave signal modulator, whereas in MSK modulation only sinusoidal shaped waves are applied to the carrier wave signal modulator. For sinusoidal wave modulation, more signal energy in the modulation signal is closer to the carrier wave signal, resulting in an opportunity for greater bandwidth efficiency. Further, MSK modulation theoretically provides a constant envelope signal that results in less degrading amplitude modulation to phase modulation conversion at the output of typical nonlinear amplifiers. Other distinctions between MSK modulation and SQPSK modulation are known to those skilled in the art.
FIG. 1 depicts a known MSK modulator 10, sometimes referred to as a "Brady" type modulator. The modulator 10 includes a mixer 12 that mixes a sinusoidal one-half clock signal and a sinusoidal carrier wave signal so that the one-half clock signal modulates the carrier wave. The modulated carrier wave signal is then applied to a diplexer 14 that acts as a tuned filter to separate the two frequencies comprising the modulated carrier wave signal. The separated frequencies from the diplexer 14 are then applied to a magic-T circuit 16 that includes a specialized configuration of phase and amplitude equalizers (PAE) 18 that processes the I and Q data rail signals to get signals having highly controlled phase and amplitude. The I data rail modulated carrier wave is then applied to an I data rail mixer 20 that mixes the I data rail modulated carrier wave with a square wave I digital data signal. The Q data rail modulated carrier wave is then applied to a Q data rail mixer 22 that mixes the Q data rail modulated carrier wave with a square wave Q digital data signal. The modulated I and Q digital data signals from the mixers 20 and 22 are then applied to a summer 24 that sums the signals in phase. An output signal S(t) of data and clock signals modulated onto a carrier wave signal is emitted from the modular 10 to a transmitter.
FIG. 2 depicts another MSK modulator 30, sometimes referred to as a "Collins" modulator. An example of this type of MSK modulator can be found in U.S. Pat. No. 4,648,098 issued to Kerr. In this version, a one-half clock signal is applied to a first 90.degree. hybrid coupler 32 to separate the clock signal into two clock signals that are out of phase with each other by 90.degree.. A local oscillator signal acting as a carrier wave signal is applied to a second 90.degree. hybrid coupler 34 to separate the local oscillator signal into two signals that are 90.degree. apart in phase. The zero phase signals from the coupler 32 and the coupler 34 are mixed in a mixer 36 to create a modulated carrier wave signal for an I data rail. The 90.degree. phase signals from the coupler 32 and the coupler 34 are mixed in a mixer 38 to create a modulated carrier wave signal for a Q data rail. The zero phase I data rail signal is then applied to a series of PAEs 40, and the 90.degree. phase Q data rail signal is then applied to a series of PAEs 42 so that the I and Q data rail signals are substantially equal in amplitude and are separated by 90.degree.. The I data rail signal from the PAEs 40 is applied to a mixer 44 to be mixed with a square wave I digital data signal, and the Q rail signal from the PAEs 42 is applied to a mixer 46 to be mixed with a square wave Q digital data signal. The I data signal modulated onto the carrier wave and the Q data signal modulated onto the carrier wave are then summed in a summer 48 to be transmitted as a signal S(t).
As is apparent from the discussion of the prior art modulators 10 and 30 above, the clock signal modulates the carrier wave prior to the digital data signals being impressed upon the carrier wave. For this and other reasons, the prior art MSK modulators suffer from a number of disadvantages. Particularly, the modulator 10 requires extensive amplitude/phase balancing in the magic-T circuit 16 that is different for each different data rate. Further, the modulator 10 may require heaters to compensate for temperature variations in the temperature sensitive diplexer 14. Also, the modulator 10 requires significant data/clock signal alignment and a different diplexer for each different data rate, and is not readily configured into monolithic microwave integrated circuit (MMIC) technology. The modulator 30 requires difficult high frequency linear modulation development with high local oscillator/RF isolation to remove local oscillator leakage in the mixers 36 and 38. Also, the modulator 30 requires amplitude/fade balance tuning, significant data/clock signal alignment and different BPFs for different data rates. Other types of MSK modulators known in the art suffer from these and other disadvantages.
What is needed is an MSK modulator that provides high data rate transmission, but does not suffer from the above and other drawbacks of known MSK modulators. It is therefore an object of the present invention to provide such a modulator.